separate action

.github/workflows/Lean_decl_diff.yaml

@@ -0,0 +1,20 @@
+on:
+  workflow_run:
+    workflows: ["Build"]
+    types:
+      - completed
+
+  Lean_decl_diff:
+    if: github.repository MAIN_OR_FORK 'leanprover-community/mathlib4'
+    name: summarize_declarations
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ## fetch the whole repository, useful to find a common fork
+          fetch-depth: 0
+      - name: Lean-diff
+        run: |
+          git checkout master
+          git checkout -
+          ./scripts/decls_diff_hybrid.sh "${{ github.sha }}" "$(git merge-base master ${{ github.sha }})"